1. Field of the Invention
The present invention relates to a method of forming solder bumps. More particularly, the invention relates to a method of forming solder bumps that realizes formation of solder bumps with a desired height on electrodes or pads of an object to be connected, such as a substrate, a LSI (Large-Scale Integration) bare chip, and an electronic component, while suppressing or preventing generation of the defect of voids in solder bumps formed.
2. Description of the Related Art
The xe2x80x9cflip-chip bondingxe2x80x9d method, which is known well, is typically used to interconnect a LSI bare chip (which may be termed a xe2x80x9cLSI chipxe2x80x9d or xe2x80x9cbare chipxe2x80x9d, hereinafter) to a mounting substrate. In this method, xe2x80x9csolder bumpsxe2x80x9d (which are typically approximately spherical projections of solder) are formed on the electrodes or bonding pads of a LSI bare chip. Then, the solder bumps are opposed and joined to the corresponding electrodes or pads of a mounting substrate, thereby connecting electrically and mechanically the bumps on the chip to the corresponding electrodes or pads of the substrate. Thus, the bare chip is fixed or mounted on the substrate in its face-down state.
The flip-chip bonding method may be applicable to mounting and connection of other electronic elements and/or components than LSI bare chips. In this case, the solder bumps are formed on the electrodes (i.e., lands) of a mounting board (e.g., a Printed Wiring Board (PWB)) or the terminals or electrodes of an electronic element or component.
Conventionally, various types of method of forming solder bumps have been known, which are divided into several groups corresponding to the technique used therein. For example, one of the groups uses a solder layer formed by a plating or evaporating process of a solder material. Another one of the groups uses the placement of solder balls on the electrodes. Still another one of the groups uses a solder paste layer formed by a printing process. Recently, there is a method using xe2x80x9cmetal jetxe2x80x9d. Solder layers or solder balls, which are formed or placed on the electrodes by one of these methods, are temporarily melted due to heat in a reflowing furnace, forming molten solder pieces on the electrodes. These molten solder pieces are likely to be round or projective due to their surface tension and thus, the molten solder pieces are cooled to solidify naturally as they are. As a result, approximately spherical or projective solder bumps are formed on the electrodes.
These types of method have their own advantages and disadvantages and therefore, it is usual that one of them is selected and used according to the purpose.
From the viewpoint of fabrication cost, it is most preferred to select the type of method using a printed solder paste layer as preliminary solder pieces. This is because the preliminary solder pieces are formed by a screen printing process and thus, the method can be conducted with simple facilities and the fabrication cost can be reduced easily.
However, the type of method using a solder paste layer has a disadvantage that an obtainable height of solder bumps is smaller than the other types. This is due to the fact that the amount of solder to be placed on each electrode or pad has to be limited to a low level through one printing process in order to prevent the defect termed xe2x80x9csolder bridgexe2x80x9d and xe2x80x9cdeviation or fluctuation of solder amountxe2x80x9d. On the other hand, if the height of solder bumps is insufficient, defective connection of the solder bumps to the corresponding electrodes or pads tends to occur because of the surface irregularity and/or thermal deformation of the substrate. Accordingly, some contrivance is necessary to realize solder bumps with desired, sufficient height without any defects.
The type of method using a solder paste layer has another disadvantage that a defect termed xe2x80x9cvoidxe2x80x9d is likely to be formed in solder bumps. This is because a solder paste is mainly made of solder particles and a flux and therefore, it has less wettability to the electrodes or pads than that of the other types of method. If a void occurs in a solder bump, the effective connection area of the bump to the electrode or pad decreases largely and at the same time, stress concentration occurs in the neighborhood of the void. As a result, there arises a serious problem of degradation of mounting or interconnection reliability.
As explained above, with the conventional method of forming solder bumps using screen printing, some measure needs to be taken to form void-free, good-quality solder bumps with a sufficient height while making use of the advantage of low cost. To provide such measure, various improvements have ever been studied and developed for this method.
The Japanese Non-Examined Patent Publication No. 11-40936 published in 1999 discloses an improved method of forming solder bumps. In this method, a patterned solder paste layer is formed on a substrate by printing using a metal mask (i.e., the printing step) and then, the solder paste layer is reflown (i.e., the reflowing step). The printing and reflowing steps are repeated according to the necessity to obtain solder bumps with a desired height. As a result, even if conventional mounting facilities are used, there arises no defects such as xe2x80x9csolder bridgexe2x80x9d caused by the excessive openings of the mask and/or xe2x80x9csolder amount dispersionxe2x80x9d caused by bad detachment behavior of the mask from the substrate due to the excessive thickness of the mask. This means that the resultant solder bumps have a desired height and are uniform in size.
In the prior-art method disclosed by the Publication No. 11-40936, if necessary, when the combination of the printing and reflowing steps is repeated, the thickness of the metal mask and/or the diameter and/or shape of the openings of the mask may be changed. Moreover, the composition and/or characteristics of the solder paste used may be varied, and the process condition of the printing or reflowing step may be changed, according to the necessity.
The Japanese Non-Examined Patent Publication No. 11-145176 published in 1999 discloses another improved method of forming solder bumps using screen printing. In this method, a patterned solder paste layer is formed by printing on a substrate using a mask with openings (i.e., the printing step) and then, the solder paste layer is heated and melted temporarily, forming a solder bump layer (the heating step). The combination of the printing and heating steps is defined as a xe2x80x9cbump layer formation cyclexe2x80x9d. The bump layer formation cycle is repeated as necessary to form solder bumps with a desired height.
In the prior-art method of forming solder bumps disclosed by the Publication No. 11-145176, if necessary, a flattening step of flattening the upper face of the solder bump layer may be added. When the bump layer formation cycle is repeated, the thickness of the mask, the property and/or fluidity of the solder paste used, or the area or size of the openings of the mask may be changed.
The above-described Publication No. 11-145176 further discloses a method of forming preliminary solder pieces. This method is used to form preliminary solder pieces on a mounting substrate on which an object (e.g., a LSI chip) with solder bumps is to be mounted by the flip-chip bonding method. A resist film with openings at the connection points for the solder bumps is formed on the substrate. The preliminary solder pieces are placed at the connection points. In this method, a patterned solder paste layer is formed on the substrate by printing using a mask with openings at the connection points (i.e., the printing step) and then, the solder paste layer is heated and melted temporarily (i.e., the heating step), thereby forming preliminary solder pieces on the pads of the substrate. The openings of the mask used in the printing step are set to be larger in size or area than those of the resist film.
In the prior-art method forming preliminary solder pieces disclosed by the Publication No. 11-145176, the preliminary solder pieces are placed on the pads of the substrate on which an LSI chip is to be mounted. The preliminary solder pieces are formed to enhance the electrical connectivity between the pads on the substrate and the solder bumps of the LSI chip.
With the prior-art method of forming preliminary solder pieces disclosed by the Publication No. 11-145176, the openings of the mask used in the printing step are set to be larger in size or area than the resist film and therefore, the detachment behavior of the mask from the substrate after the printing step is finished is improved. As a result, the solder paste is prevented from being attached to the mask when the mask is detached from the substrate. This ensures the formation of the preliminary solder pieces with a desired shape.
In the prior-art method of forming preliminary solder pieces disclosed by the Publication No. 11-145176, a pressing step of pressing the solder paste filled into the openings of the resist film and those of the mask may be added. In this case, there is an advantage that the filling or packing property of the solder paste is improved. Moreover, if the diameter of the solder particles in the paste and the diameter of the openings of the mask have a specific relationship, there is an advantage that the filling or packing property of the solder paste is improved. When the mask is attached onto the substrate in the printing step, the mask may be located in such a way that the openings of the mask are shifted from their concentric positions with respect to the openings of the resist film. In this case, there is an advantage that the void defect is prevented from being formed in the preliminary solder pieces in the heating step.
As described above in detail, with the conventional method of forming solder bumps using screen printing, there is a problem that an obtainable height of solder bumps is insufficient and the void defect tends to occur in solder bumps, and that obtainable electrical and mechanical interconnection reliability is insufficient.
With the prior-art method of forming solder bumps disclosed by the Publication No. 11-40936, to obtain solder bumps with a desired height, the printing and reflowing steps are repeated according to the necessity to thereby place a specific amount of the solder paste on the substrate step by step. As a result, even if conventional mounting facilities are used, there arises no defect such as xe2x80x9csolder bridgexe2x80x9d and/or xe2x80x9csolder amount dispersionxe2x80x9d, resulting in desired height and uniform size of the solder bumps. However, this method provides no consideration on prevention of the void defect that are likely to occur in solder bumps.
With the prior-art method of forming solder bumps disclosed by the Publication No. 11-145176, like the prior-art method disclosed by the Publication No. 11-40936, there is no consideration on prevention of the void defect that are likely to occur in solder bumps.
With the prior-art method of forming preliminary solder pieces disclosed by the Publication No. 11-145176, when the mask is attached or placed onto the substrate in the printing step, the mask may be located in such a way that the openings of the mask are shifted from their concentric positions with respect to the openings of the resist film. In this case, there is an advantage that the void defect is prevented from being formed in the preliminary solder pieces in the heating step. However, the disclosed reason for this advantage is that the air existing in minute holes or depressions generated by the surface irregularity of each pad is confined by the solder paste. If the mask is placed to be shifted that way, the air will be moved out of the holes or depressions when the molten solder flows onto the pad, thereby preventing the void defect.
With the prior-art method of forming preliminary solder pieces of the Publication No. 11-145176, however, the resist film with the openings needs to be formed on the mounting substrate and at the same time, the mask with the openings needs to be placed on the resist film. Moreover, the openings of the mask have to be shifted with respect to those of the resist film as described above. As a result, the method has a problem that it is unable to be applied to the formation of solder bumps on the electrodes of a LSI chip, and that the process itself of forming preliminary solder pieces is complicated. Additionally, this method is a method of forming preliminary solder pieces, not solder bumps.
Accordingly, an object of the present invention is to provide a method of forming solder bumps that makes it possible to form solder bumps with desired height while suppressing or preventing generation of the defect of voids in solder bumps.
Another object of the present invention is to provide a method of forming solder bumps that facilitates the prevention of the void defect in the solder bumps with a simple measure.
The above objects together with others not specifically mentioned will become clear to those skilled in the art from the following description.
According to a first aspect of the invention, a method of forming solder bumps is provided, which comprises the steps of:
(a) forming first solder paste layers on respective electrodes or pads of a substrate by printing a solder paste to have a specific pattern on the electrodes or pads using a first mask with openings;
(b) forming first solder bumps on the respective electrodes or pads of the substrate by melting the first solder paste layers due to heat for a while and by solidifying the first solder paste layers thus melted, after removing the first mask from the substrate;
(c) forming second solder paste layers on the respective first solder bumps over the electrodes or pads of the substrate by printing a solder paste to have a specific pattern on the first solder bumps using a second mask with openings; and
(d) forming second solder bumps on the respective electrodes or pads of the substrate by melting the first solder bumps and the second solder paste layers due to heat for a while to be integrated together and by solidifying the first solder bumps and the second solder paste layers thus melted and integrated, after removing the second mask from the substrate;
the second solder bumps being larger than the first solder bumps;
wherein each of the openings of the first mask has a volume in such a way that each of the first solder bumps has a height approximately equal to maximum size of a void to be suppressed according to size of a corresponding one of the electrodes or pads.
With the method according to the first aspect of the invention, the above-described objects are achieved in the following way:
The inventor found, after his thorough investigation, that the main cause of the void defect in solder bumps is gaseous substances generated by reaction between electrodes/pads of a substrate and a solder paste placed thereon in the heating and reflowing process. In other words, he found that when molten solder paste pieces formed on the electrodes/pads solidify to become solder bumps after the heating and reflowing process, the gaseous substances thus generated are likely to be confined in these bumps. As a result, hollow spaces or cavities are formed within the bumps on the electrodes/pads. These spaces or cavities will be defective voids.
Furthermore, the inventor found that the more similar to a sphere the molten solder pieces are, the easier the gaseous substances are confined in the resultant bumps (i.e., the higher the possibility of voids becomes), and that the more similar to a sphere the pieces are, the larger the possible size or diameter of voids grows. This means that the more dissimilar to a sphere (the more similar to a plane) the pieces are, the lower the possibility of voids becomes and at the same time, the smaller the possible size of voids is. The invention was created based on this knowledge.
With the method according to the first aspect of the invention, each of the openings of the first mask has a volume in such a way that each of the first solder bumps has a height approximately equal to maximum size of a void to be suppressed according to size of a corresponding one of the electrodes or pads. Therefore, even if hollow spaces or cavities are formed within the first solder bumps on the respective electrodes/pads, the spaces or cavities that are approximately equal to the maximum size of the voids to be suppressed will burst and disappear easily.
There is a possibility that hollow spaces or cavities that are smaller than the maximum size of the voids to be suppressed are left in the first solder bumps. However, if the maximum size of the voids to be suppressed is set to be a suitable value, these smaller spaces or cavities can be dealt so as not to raise any practical problem.
As described above, the gaseous substances are generated by the reaction between different metallic materials (i.e., the electrodes and the solder paste) in the step (b) of forming the first solder bumps. Therefore, it can be said that such gaseous substances are not substantially generated in the step (d) of forming the second solder bumps, because the first solder bumps and the second solder paste layers (which are made of almost the same material) are melted and integrated together in the step (d). As a result, there is no limitation of the height of the second solder bumps formed in the step (d).
Accordingly, with the method of the first aspect of the invention, solder bumps with desired height can be formed on the respective electrodes/pads while suppressing or preventing generation of the defect of voids in the solder bumps. Moreover, since this method requires no special, additional process and no special printing condition of solder paste, it facilitates the prevention of the void defect in the solder bumps with a simple measure.
In addition, needless to say, the second solder paste layers may be contacted with the electrodes/pads at their side faces in the step (d). However, taking the shape and/or structure of the electrodes/pads into consideration, the possible contact areas between the electrodes/pads and the second solder paste layers in the step (d) are restricted within a sufficiently low level. As a result, even if a trace of gaseous substance is generated in the step (d), it can be ignored.
In a preferred embodiment of the method according to the first aspect, the first mask is thicker than the electrodes or pads of the substrate while the openings of the first mask are larger than the electrodes. Moreover, the second mask is thicker than the first mask while the openings of the second mask are larger than the corresponding openings of the first mask. In this embodiment, the advantages of the invention are more conspicuous.
In another preferred embodiment of the method according to the first aspect, the second solder bumps cover not only top faces of the respective electrodes or pads but also side faces thereof.
In still another preferred embodiment of the method according to the first aspect, each of the openings of the second mask has a volume in such a way that each of the second solder bumps has a height equal to a desired value.
According to a second aspect of the invention, another method of forming solder bumps is provided, which comprises the steps of:
(a) forming first solder paste layers on respective electrodes or pads of a substrate by printing a solder paste to have a specific pattern on the electrodes or pads using a first mask with openings;
(b) forming first solder bumps on the respective electrodes or pads of the substrate by melting the first solder paste layers due to heat for a while and solidifying the first solder paste layers thus melted, without removing the first mask from the substrate;
(c) forming a second mask with openings on the first mask;
(d) forming second solder paste layers on the respective first solder bumps over the electrodes or pads of the substrate by printing a solder paste to have a specific pattern on the first solder bumps using the second mask;
(e) forming second solder bumps on the respective electrodes or pads of the substrate by melting the first solder bumps and the second solder paste layers due to heat for a while to be integrated together and by solidifying the first solder bumps and the second solder paste layers thus melted and integrated, without removing the first and second masks from the substrate;
the second solder bumps being larger than the first solder bumps; and
(f) removing the second and first masks from the substrate after the step (e);
wherein each of the openings of the first mask has a volume in such a way that each of the first solder bumps has a height approximately equal to maximum size of a void to be suppressed according to size of a corresponding one of the electrodes or pads.
With the method according to the second aspect of the invention, each of the openings of the first mask has a volume in such a way that each of the first solder bumps has a height approximately equal to maximum size of a void to be suppressed according to size of a corresponding one of the electrodes or pads. Therefore, even if hollow spaces or cavities are formed within the first solder bumps on the respective electrodes/pads, the spaces or cavities that are approximately equal to the maximum size of the voids to be suppressed will burst and disappear easily.
There is a possibility that hollow spaces or cavities that are smaller than the maximum size of the voids to be suppressed are left in the first solder bumps. However, if the maximum size of the voids to be suppressed is set to be a suitable value, these smaller spaces or cavities can be dealt so as not to raise any practical problem.
As described above, the gaseous substances are generated by the reaction between different metallic materials (i.e., the electrodes and the solder paste) in the step (b) of forming the first solder bumps. Therefore, it can be said that such gaseous substances are not substantially generated in the step (e) of forming the second solder bumps, because the first solder bumps and the second solder paste layers (which are made of almost the same material) are melted and integrated together in the step (e). As a result, there is no limitation of the height of the second solder bumps formed in the step (e).
Accordingly, with the method of the second aspect of the invention, solder bumps with desired height can be formed on the respective electrodes/pads while suppressing or preventing generation of the defect of voids in the solder bumps. Moreover, since this method requires no special, additional process and no special printing condition of solder paste, it facilitates the prevention of the void defect in the solder bumps with a simple measure.
In addition, needless to say, the second solder paste layers may be contacted with the electrodes/pads at their side faces in the step (e). However, taking the shape and/or structure of the electrodes/pads into consideration, the possible contact areas in between the electrodes/pads and the second solder paste layers in the step (e) are restricted within a sufficiently low level. As a result, even if a trace of gaseous substance is generated in the step (e), it can be ignored.
In a preferred embodiment of the method according to the second aspect, the first mask is thicker than the electrodes or pads of the substrate while the openings of the first mask are larger than the corresponding electrodes or pads. Moreover, the openings of the second mask are approximately equal in size to the corresponding openings of the first mask.
In another preferred embodiment of the method according to the second aspect, the second solder bumps cover not only top faces of the respective electrodes or pads but also side faces thereof.
In still another preferred embodiment of the method according to the second aspect, sum of each of the openings of the second mask and a corresponding one of the openings of the first mask has a volume in such a way that each of the second solder bumps has a height equal to a desired value.
According to a third aspect of the invention, still another method of forming solder bumps is provided, which comprises the steps of:
(a) placing first solder balls on respective electrodes or pads of a substrate;
(b) forming first solder bumps on the respective electrodes or pads of the substrate by melting the first solder balls due to heat for a while and by solidifying the first solder balls thus melted;
(c) placing second solder balls on the respective first solder bumps over the electrodes or pads of the substrate; and
(d) forming second solder bumps on the respective electrodes or pads of the substrate by melting the first solder bumps and the second solder balls due to heat for a while to be integrated together and by solidifying the first solder bumps and the second solder balls thus melted and integrated;
the second solder bumps being larger than the first solder bumps;
wherein each of the first solder balls has a volume in such a way that each of the first solder bumps has a height approximately equal to maximum size of a void to be suppressed according to size of a corresponding one of the electrodes or pads.
With the method according to the third aspect of the invention, each of the first solder balls has a volume in such a way that each of the first solder bumps has a height approximately equal to maximum size of a void to be suppressed according to size of a corresponding one of the electrodes or pads. Therefore, even if hollow spaces or cavities are formed within the first solder bumps on the respective electrodes/pads, the spaces or cavities that are approximately equal to the maximum size of the voids to be suppressed will burst and disappear easily.
There is a possibility that hollow spaces or cavities that are smaller than the maximum size of the voids to be suppressed are left in the first solder bumps. However, if the maximum size of the voids to be suppressed is set to be a suitable value, these smaller spaces or cavities can be dealt so as not to raise any practical problem.
As described above, the gaseous substances are generated by the reaction between different metallic materials (i.e., the electrodes and the solder balls) in the step (b) of forming the first solder bumps. Therefore, it can be said that such gaseous substances are not substantially generated in the step (d) of forming the second solder bumps, because the first solder bumps and the second solder balls (which are made of almost the same material) are melted and integrated together in the step (d). As a result, there is no limitation of the height of the second solder bumps formed in the step (d).
Accordingly, with the method of the third aspect of the invention, solder bumps with desired height can be formed on the respective electrodes/pads while suppressing or preventing generation of the defect of voids in the solder bumps. Moreover, since this method requires no special, additional process and no special printing condition of solder paste, it facilitates the prevention of the void defect in the solder bumps with a simple measure.
In a preferred embodiment of the method according to the god third aspect, the second solder balls are larger in diameter than the corresponding electrodes or pads of the substrate.
In another preferred embodiment of the method according to the third aspect, the second solder bumps cover not only top faces of the respective electrodes or pads but also side faces thereof.
In still another preferred embodiment of the method according to the third aspect, each of the second solder balls has a volume in such a way that each of the second solder bumps has a height equal to a desired value.